Does the ruling on sorrow and regret for missing out on something due to personal negligence differ from that concerning calamities that befall a person without their causing them? And is this considered an objection to divine decree?

The monotheistic believer must believe that everything that befalls him is by the decree of Allah Almighty, and that he should not grieve over what he has missed. This is because what has befallen him is either due to a sin he committed, in which case he must repent and seek forgiveness, or it is a trial to raise his ranks and expiate his sins, in which case he must be content and patient. The Prophet, peace and blessings be upon him, forbade saying "if only" when calamities strike, because it opens the door to the work of Satan and indicates dissatisfaction with the divine decree. Rather, one should say: "Allah has decreed and what He willed, He has done."